Precambriano mollendoRegional-scale Grenvillian-age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

7/26/2019 Precambriano mollendoRegional-scale Grenvillian-age UHT metamorphism in the MollendoCamana block (basem

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Regional-scale Grenvillian-age UHT metamorphismin the MollendoCamana block (basement of the Peruvian Andes)

J . M A R T I G N O L E A N D J . - E . M A R T E L A T *

Departement de geologie, Universitede Montreal, CP 6128, Montreal, H3C 3J7, QC, Canada ([email protected])

AB ST R ACT The MollendoCamana Block (MCB) is a 50 150 km Precambrian inlier of the Andean belt thatoutcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproter-ozoic heritage intensely metamorphosed during the Grenville event (c.1 Ga; U-Pb and U-Th-Pb ages onzircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic

leucosomes (KFMASH), contain various amounts of K-feldspar (Kfs), orthopyroxene (XMgOpx 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite(Mag), quartz (Qtz), and minor amounts of garnet (XMg Grt 0.60), sapphirine (XMg Spr 0.87),cordierite (XMg Crd 0.92) and biotite (XMg Bt 0.83). The ubiquitous peak mineral assemblage isOpx-Sil-Kfs-Qtz-( Grt) in most of the MCB, which, together with the high Al content of

orthopyroxene (10% Al2O3) and the local coexistence of sapphirine-quartz, attest to regional UHTmetamorphism (> 900 C) at pressures in excess of 1.0 GPa. Fluid-absent melting of biotite isresponsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (andmost of the garnet) in the southern part of the MCB (Mollendo-Cocachacra areas). In this area, a firststage of decompression from 1.11.2 to 0.80.9 GPa at temperatures in excess of 950 C, is marked bythe breakdown of Sil-Opx to Spr-Opx-Crd assemblages according to several bivariant FMAS reactions.High-T decompression is also shown by Mg-rich garnet being replaced by Crd-Spr- and Crd-Opx-bearing symplectites, and reacting with quartz to produce low-Al-Opx-Sil symplectites in quartz-rich

migmatites. Neither osumilite nor spinel-quartz assemblages being formed, isobaric cooling at about 0.9GPa probably followed the initial decompression and proceeded with massive precipitation of meltstowards the (Os) invariant point, as demonstrated by Bt-Qtz-( pl) symplectites in quartz-rich

migmatites (melt + Opx + Sil Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondarybiotite attest to late fluid-absent melting, compatible with a second stage of decompression below900 C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas

the regional extent of UHT metamorphism in the area, probably results from large-scale penetration ofhot asthenospheric mantle at the base of an over-thickened crust.

Key words: Arequipa; Grenville; Peru; sapphirine; UHT metamorphism.

I NT R O DUCT I O N AND GEO LO GI CAL SET T I NG

Precambrian inliers form the backbone of the SouthAmerican cordilleras (Fig. 1) from Columbia (Irwin,1975; Restrepo-Pace et al., 1997), to northern Chile(Pacciet al., 1981). A connection with the western partof the Amazon craton has been suggested for some ofthese inliers, based on the similarity of plutonic suites

(e.g. Sadowski & Bettencourt, 1996) and isotopicsignatures (e.g. Litherland et al., 1989; Tosdal, 1996).In Peru, Precambrian rocks are found in the orientalcordillera (Audebaud et al., 1971), in the occidentalcordillera, north-west of Arequipa (Charcani gneisses)and near the towns of Omate, Moquegua andTacna. Along the Pacific coast, Precambrian rocksoutcrop from Paracas to Ilo (Dalmayrac et al., 1977;Shackleton et al., 1979). Shackleton et al. (1979)

coined the term Arequipa Massif for a block of Pre-cambrian gneisses that outcrops along the Pacific coastfor 800 km and extends inland for c. 100 km. Theysubdivided this massif into (nonlimited) domains, threeof which are more or less similar in their high meta-morphic grade, namely Mollendo, Quilca and Camana(Fig. 2). They noted the ubiquitous presence of silli-manite, the occurrence of the assemblage ortho-pyroxene-sillimanite-quartz, the exceptional occurrenceof sapphirine near Mollendo, in rocks that are other-wise described as migmatites, those from the Quilcaarea being more pegmatite-rich. Shackleton et al.(1979) attributed these parageneses to a M1 (Precam-brian) metamorphism. These high-grade areas areseparated from nonmigmatitic, muscovite-rich schistsand migmatitic amphibolites of the Ocon a Marconasegment (Fig. 1) by a phyllonite zone.

Several attempts at dating rocks of the Arequipamassif have been made since the preliminary RbSrisochron of 1012 52 Ma (Charcani gneisses of

* Now at: Laboratoire de Ge odynamique des Chanes alpines (UMR

5025)BP 53, 38041 GRENOBLE Cedex, France.

J. metamorphic Geol., 2003,21, 99120

Blackwell Science Inc., 0263-4929/03/$15.00 99Journal of Metamorphic Geology, Volume 21, Number 1, 2003


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James & Brooks, 1976). A RbSr whole-rock K-feld-

spar isochron on a granulite from the Mollendo areayielded an age of 1811 39 Ma with an initial87Sr86Sr of 0.7086 0.0009, an age interpreted asthat of high-grade metamorphism (Cobbing et al.,1977). In the same area, another RbSr isochron basedon whole rock gave 1918 33 Ma with an 87Sr86Srinitial ratio of 0.704 0.002 (Shackleton et al., 1979).A preliminary U-Pb zircon plot from a granulite of theMollendo area gave a 207Pb206Pb age of c. 1950 Mawith a potential episode of lead loss at c. 600 Ma(Dalmayrac et al., 1977). More recently Wasteneyset al. (1995) based on U-Pb determinations from zir-con concluded that two domains with different meta-morphic ages exist within high-grade rocks along thecoast of Peru: the first one located north of Quilca anddated atc.1198 4 Ma, the second one located northof Mollendo and dated at c. 970 Ma. Inherited zirconin both domains suggests a c. 1900 Ma age for theprotolith of the Arequipa massif.

The aim of this contribution is to present newgeochronological, structural, mineralogical and petro-logical data on granulite-grade rocks, ultra-hightemperature assemblages (UHT; T >900 C) andassociated migmatites that occupy the portion ofthe Arequipa massif located between the towns of

Mollendo and Camana (Fig. 2). This area will bereferred to here as the MollendoCamana block (MCB)andwill be subdivided into the Camana, Quilca, Arantasand Pampa Blanca areas (northern part of MCB) andthe Mollendo and Cocachacra areas (southern part ofthe MCB). This will add to the database for the fewlocalities around the world where UHT metamorphic

Fig. 2. Foliation trajectories in the MollendoCamana Block.Stereonets (lower hemisphere projection): poles to metamorphicfoliations (black dots); mineral lineations (white stars): (1) Pre-cambrian migmatites; (2): Precambrian (undated) granites;(3): Devonian to Carboniferous sediments; (4): Phanerozoicsediments; (5): Phanerozoic intrusive rocks (adapted from geo-

logical maps of the Servicio de Geologa y Minera (scale 1 :100 000; Yesira, Aplao, Arequipa, Camana, Mollendo, La Joya,Punta de Bonbo n).

Fig. 1. Precambrian inliers in the Andean belt of Peru (modifiedfrom Shackleton et al., 1979 and the tectonic map of SouthAmerica, Almaida et al., 1978); (1):Undifferentiated Precam-brian rocks; (2): Proterozoic granulites; (3): Proterozoic green-schist to amphibolite facies rocks; (4): Phanerozoic sediments;(5): Mid-Palaeozoic to Triassic batholiths; (6): Andean batho-liths; (7): Upper Cretaceous to Quaternary effusive rocks.

1 0 0 J . M A R T I G N O L E & J . - E . M AR T E L A T


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rocks are well exposed such as in Antarctica (e.g. Harley,1998a and reference therein), Eastern Ghats (e.g. Grew,1982), Hoggar (e.g. Kienast & Ouzegane, 1987), ItalianAlps (Droop & Bucher-Nurminen, 1984) and Scotland(Baba, 1999). Quantitative pressure and tempera-ture estimates and P-Ttrajectories of Peruvian UHTmigmatites will be dealt with in a separate paper.

ST R UCT UR E

The high-grade migmatitic MollendoCamana Block(MCB) outcrops mainly along the shore of the Pacific,but also in deep V-shaped valleys that dissect thepampa, and on some hills rising a few hundred metresabove the pampa (Fig. 2). However, even in areas ofcontinuous outcrop, the highly monotonous lithologi-cal succession makes difficult the identification andmapping of large-scale structural elements.

Most rocks of the MCB are foliated migmatiteswith interlayered, more homogeneous gneisses. In the

migmatites, the foliation is marked by the separa-tion of irregular, alternating cm-scale light-colouredquartzofeldspathic (leucosomes) and dark sillimanite-orthopyroxene-( oxides) layers (mesosomes). Quart-zofeldspathic layers may be discontinuous but rarelyanastomosing although they may constitute as muchas 50% of the rock (stromatitic migmatites; Fig. 3).Locally, layering is more conspicuous with thinner lay-ers, suggesting higher strain. Foliations are dominantlyoriented E-W to NWSE with varying dips (Fig. 2). Themost common linear fabric is defined by the alignmentof sillimanite prisms and is generally striking E-W in theMollendo area and N-S in the Quilca area.

Cm- to m-scale folds are found in most outcrops

including several occurrences of superimposed folds.Fold axes are dominantly E-W in the Mollendo area,namely parallel to the mineral lineation. Variations infoliation strikes and dips and in foliation trends at mapscale suggest the presence of regional-scale folds thatcannot be confidently traced due to the absence ofmarker horizons.

P ET R O GR AP HY

The main rock types in the MCB are migmatites (Fig. 3),aluminous gneisses, rare mafic granulites and granites.Pegmatite pods, aplitic sills and dykes, are common inthe Quilca and Camana areas but rare in the Mollendoarea. Most outcrops of the MCB can be described asgenuine migmatites with well-segregated leucosomesand aluminous pods or layers that can be describedas mesosomes. More homogeneous outcrops that lackthe stromatitic structure of most migmatites arereferred to as aluminous gneisses due to the ubiquitouspresence of sillimanite. Undated muscovite pegmatites,Phanerozoic granites, and Mesozoic diabase dykesintersect the lithological units of the MCB. This con-tribution will be concerned essentially with the migma-tites and the intercalated aluminous gneisses.

Migmatites

Leucosomes

Leucosomes consist of light, coarse-grained aggregates(Fig. 4) of mesoperthite, perthitic orthoclase ormicrocline (up to 60%), quartz (up to 50%), plagio-

clase (1020%), with minor and variable amounts oforthopyroxene, sillimanite, oxides, biotite and traces ofsapphirine. Grain size is usually larger than 1 mmespecially concerning the orthopyroxene that is muchlarger than that in the mesosomes.

Mesosomes

The mesosomes are comprised of the following min-erals: sillimanite (5575%), orthopyroxene (015%),garnet (010%), cordierite (020%), magnetite-ilmen-ite (210%), biotite (010%), sapphirine and spinel(010%). Grain size ranges from 100 to 1500 lm.

Isolated prismatic sillimanite occurs within subinter-stitial oxide grains (Fig. 5; sample 15, see Fig. 6 forlocation). Monomineralic sillimanite aggregates con-taining several tens of grains, with interstitial oxides,may reach cm-scale. Nests of strongly pleochroic pinkto light green orthopyroxene surround sillimaniteaggregates whereas isolated grains are interstitial orassociated with oxides. Small idiomorphic, inclusion-free garnet and trails of inclusion-rich garnet occurwithin the matrix or at the interface between themesosome and the leucosome. Oxides (mainly mag-netite) constitute up to 10% of the mesosome especi-ally in sillimanite-oxide layers.

Aluminous and semialuminous gneisses

Aluminous gneisses are devoid of the migmatitic habitalthough their mineral composition is similar to that ofthe migmatites. Grain size is intermediate between thatof leucosome and mesosome and it is likely that somelayers of aluminous gneisses are in fact thick meso-somes. Semi-aluminous (sillimanite-free) gneisses areless common and preferentially located in the Camanaarea. They consist of quartz (1530%), feldspars(orthoclase and plagioclase; 5060%), orthopyroxene(010%), garnet (010%), magnetite-ilmenite (05%),biotite (02%), spinel (010%), with accessory zirconand monazite.

Arrested reactions features are common in both themesosomes and the aluminous gneisses. They involvegarnet, cordierite, orthopyroxene, sapphirine, sillima-nite, oxides, quartz and plagioclase. The most com-mon reaction textures are represented by cordieriterims around garnet and sapphirine-bearing pseudo-morphs after garnet or sapphirine-cordierite inter-growth at the contact between sillimanite andorthopyroxene. Reaction textures involving melts arealso common in the leucosomes or at the leucosomemesosome interface.

U H T M E T A M O R P H I S M I N T H E M O L L E N D O C A M A N A B L O C K 1 0 1


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Granites

Rocks that can be described as granites, to theexclusion of Palaeozoic and Mesozoic plutons, occuras dm-scale sills, less commonly as m-scale dykes orstocks. They are relatively common in Quilca andCamana areas. They are pink, hololeucocratic, aphy-

ric, medium-grained (3 mm) rocks with a faint flowfabric defined by the alignment of sillimanite needles.Additional minerals are sparse garnet and biotite.

R O CK CHEMI ST R Y

In spite of the heterogeneity of the migmatites, attempts were madeto obtain analyses from the leucosomes, the mesosomes and alsofrom the aluminous gneisses and the granites. Some analyses werecarried out by X-ray fluorescence while others were obtained byelectron microprobe scanning of thin sections.

Analytical results are reported in Table 1 and have been pro-jected from feldspar onto AFM and AS(FM) diagrams (Fig 7a,b).

All rocks are Ca-poor and Al-rich. The mesosomes are Al-rich (upto 37 wt% Al2O3) and Si-poor (37 wt% SiO2), with a relatively

high Ti and low but variable K (up to 2.4 wt% K2O). The leuco-somes are K-rich (up to 6.8 wt% K2O) and their Al-content is suchthat they can be classified as peraluminous granites (Fig. 8). InAFM plots, most rocks are silica-saturated and have anXMg < 0.50. Such Fe-rich compositions are not compatible withthe occurrence of Mg-rich minerals like sapphirine and cordierite.In this regard, they differ from other sapphirine- and cordierite-bearing UHT granulites, much of which are silica- undersaturatedand Mg-rich (e.g. Harley et al., 1990; Raith et al., 1997). The roleof magnetite and the formation of mm-scale subassemblages will bediscussed later in an attempt to reconcile bulk compositions andmineral assemblages.

UT h P b AGE DET ER MI NAT I O NS

Given the poorly constrained age of high-grade metamorphism,

Th-U-Pb chemical age determinations on monazite were attemptedby in situ microprobe measurements using calibration and statistictreatment of Montel et al. (1996). Analyses were performed on aCameca SX100 electron microprobe with four-wavelength dispersionspectrometer detectors at the laboratoire Magmas et Volcans ofClermont-Ferrand, France, according to the procedures followed byGoncalves, 2002). An accelerating voltage of 15 kV and a beamcurrent of 150 nA were adopted. U and Th were analyzed succes-sively with a PET crystal in the same WDS detector with a countingtime of 225 s and 75 s on peak, respectively. Pb was analyzed with aLPET crystal during 300 s on peak. Standards were UO2, ThO2,vanadinite and a synthetic glass for Pb. Beam current used forstandards is 100 nA. Counting time is 50 s on peak and 20 s onbackground for UO2 and ThO2, and 300 s on peak and 100 s on

Fig. 5. Texture of an orthopyroxene-sillimanite-quartz migma-tite: note isolated prismatic sillimanite grains within large,interstitial, oxides in the lower left part of the picture; leuco-somes made of orthoclase, quartz, plagioclase and large ortho-pyroxene (e.g. upper half of picture); mesosomes are morenarrow and less abundant than the leucosomes (sample 15;Camana area).

Fig. 4. Sodium cobaltinitrite stained slab showing large amountsof K-feldspar in the leucosome; Cocachacra area.

Fig. 3. UHT migmatite showing at least 50% of quartzo-feldspathic leucosome with centimetre-scale oxide clots; Quilcaarea.



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background for PbO. Individual ages were calculated using the decayscheme of Montelet al. (1996) from the U, Th and Pb concentrationsassuming that all lead in monazite is radiogenic. The 2 r errors givenon individual ages are calculated by propagating the uncertainties onU, Th and Pb concentrations (with 95% confidence level). Calcula-tion of mean ages and associated errors (95% confidence level) is

based on least-squares modelling, which discriminates multiple agepopulations. The quality of modelling can be assessed from the meansquare weighted deviation (MSWD).

Small monazite is found in most leucocratic aluminous gneissesfrom the Camana area (samples 10, 20, 28; Fig. 6). Sample 10 con-tains eight small monazite (10 lm), on which 17 analyses were made.In sample 20, a garnet-orthopyroxene aluminous gneiss, 12 meas-urements were made on two monazite 50 lm in size. For sample 28,an orthopyroxene-sillimanite gneiss, 22 measurements were made onfive monazite ranging in size from 20 lm to 70 lm. Calculatedindividual ages from U, Th and Pb concentrations range from 1064to 956 50 Ma (Table 2). The age distribution for sample 28 allowsstatistic calculation (Montel et al., 1996) that give meaningful agesattributable to a single metamorphic event at 998 11 Ma(MSWD: 1.07; Fig. 9). Likewise, statistical ages for samples 10and 20 are, respectively, 994 14 Ma (MSWD: 1.53), and1011 18 Ma (MSWD: 1.07). The same procedure was applied tomonazite from Quilca, Mollendo, Pampa Blanca and Cocachacraarea but the results are not as straightforward. For example, 52analyses on 12 monazites (1050lm) from a migmatite of theMollendo area (sample 114) give dispersed ages with a maximumaround 919 Ma. Although the dispersion of ages may have somesignificance, the geological knowledge of the area is not sufficient toprovide a sound interpretation. Moreover, severe weathering mayalso contribute to Pb- andor U-loss, hampering data interpretation.

Based upon the already published isotopic determinations and theabove chemical ages, the MollendoCamana block appears to bedominated by a quartzofeldspathic protolith dated at about1.9 0.1 Ga that was rejuvenated around 1.0 Ga during a regionalhigh-grade metamorphic event.

MAJ O R MI NER ALS: HAB I T AND CHEMI ST R Y

Minerals were analyzed with a JEOL JXA-8900 L electronmicroprobe at McGill University with LiFH, PET, PETj and TAPanalyzing crystals, operating at 20nA specimen current, 15 kVaccelerating potential, and counting time of 20 s for major ele-ments. Stoechiometry was calculated using THEBA 7.4 software(Martignole et al., unpublished) from which Fe3+ estimates areobtained assuming charge balance. Mineral analyses are for themain phases present in migmatites, aluminous and semialuminous

gneisses; they are plotted in an AFM diagram (Fig. 10; see Fig. 6for sample locations). Abbreviations for minerals are from Kretz(1983).

Garnet

Garnet is found preferentially at the leucosomemesosome interfacealthough isolated grains are found in the leucosomes. It occurs, asmm-scale porphyroblasts (Grt I) with quartz, plagioclase, sillima-nite and biotite inclusions. In some cases, porphyroblasts are cor-roded, and even strongly resorbed residual relicts of peakassemblages. Less frequently, garnet is inclusion-free, not resorbedand could represent a late phase. Garnet was analyzed from rim torim with an average spacing of 10 lm for some well-shaped roundto elongated grains, whereas a single analysis for core and rim wasperformed in most cases. Analyzed garnet (Table 3) are almandine-

pyrope solid solution (Figs 11 & 17). Pyrope varies from 22 to60 mol%, almandine from 32 to 60 mol%. The less magnesiangarnet is from the Camana area and the most magnesian from theMollendo area (Figs 11 & 17). Grossularite and andradite contentsare < 5 mol%, the latter being in many cases more abundant thangrossularite. Spessartine content is highly variable (usually17 mol%), with very high values (> 20 mol%) in resorbed garnet.Most garnet is virtually unzoned except for minor rimward increasein almandine and decrease in pyrope. A spessartine-rich garnetfrom the Pampa Blanca area (sample 103) shows a plateau-typezoning with a rimward almandine zoning from 46 to 55%, corre-lated with a decrease in the pyrope content from 36 to 22%.Similarly, spessartine increases from 13 in the core to 22% at the

Fig. 6. Sample locations and occurrences of selected mineralsand mineral assemblages (for legend see Fig. 2).



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rim whereas the grossularite-andradite content is constant at about3%. This type of zoning suggests garnet resorption consecutive tothe formation of cordierite rims. An extreme case of resorption isfound in the Cocachacra area (sample 93; Fig. 20c,f) where residualgarnet (20 lm) occupying the centre of a cordierite-orthopyroxenesymplectite has spessartine up to 19 mol% (Fig. 11).

Fig. 8. Metaluminous-peraluminous Al-(K + Na + 2 Ca) vs.Fe + Mg + Ti diagram of Debon & Le Fort (1988).

Table 2. Monazite ages calculated from U, Th, Pb concentrationin monazite from sample 28 (Opx-Sil-Qtz leucocratic gneiss).

Analytical spot Th U Pb Age (Ma)

1 93520 774 1230 167 4410 104 991 36

2 93950 777 1050 167 4380 103 987 36

3 44850 573 2930 171 2570 83 1027 52

4 48730 593 2840 170 2640 83 992 49

5 109460 830 1640 170 5080 112 970 32

6 96570 785 1310 168 4400 104 957 34

7 49660 596 4050 174 2970 87 1025 47

8 51050 604 4220 175 3090 88 1034 46

9 62680 656 3040 172 3320 91 997 42

10 52490 609 4210 176 3120 88 1023 45

11 53220 613 4000 175 3040 88 997 45

12 50050 598 4200 175 3010 87 1024 46

13 90570 765 970 167 4190 101 981 36

14 84300 743 1180 166 3840 98 956 37

15 81870 733 1130 165 3860 97 989 39

16 52700 610 4190 176 3050 88 998 45

17 48700 594 3180 172 2790 85 1027 49

18 48280 590 2440 169 2620 83 1015 51

19 61970 652 2310 170 3210 89 1009 44

20 87990 756 2410 172 4320 103 986 36

21 45890 578 2300 168 2500 82 1020 53

22 52560 611 5000 179 3390 92 1064 45

Th, U, Pb in p.p.m.; error limits at 95% confidence level.

a b

Fig. 7. (a) AFM and (b): S(FM)A plots ofbulk compositions for analyzed rocks of theMCB (filled symbols: XRF analyses; whitesymbols, electron probe analyses; F FeOtot); sample numbers refer to those inTable 1.

Table 1. Major and trace elements of leucosomes, mesosomes and granitic rocks from Quilca, Mollendo, and Cocachacra areas ofthe MCB.

Area QU QU QU MO MO MO MO MO MO MO CO CO CO CO CO CO

Sample 242* 245* 246* 51 53a(m) 53b(m) 55 63 81 220* 93 95 95* 95(m) 95(m)* 209(m)*

SiO2 73.92 73.30 73.04 72.15 48.99 61.15 69.04 77.37 75.74 76.57 78.75 73.13 87.63 52.45 36.67 67.58

TiO2 0.04 0.08 0.06 0.77 1.20 0.81 0.67 0.47 0.60 0.14 0.60 0.26 0.05 1.45 1.97 0.89

Al2O3 14.93 15.21 14.69 14.97 36.86 25.56 16.64 10.41 12.29 13.03 13.64 14.88 6.27 31.74 37.54 16.05

FeO 0.35 0.76 0.53 5.29 10.24 7.13 5.67 3.66 3.76 1.12 4.12 1.75 0.55 11.81 16.33 7.22MnO 0.018 0.071 0.025 0.00 0.11 0.12 0.14 0.12 0.08 0.009 0.10 0.00 0.042 0.00 0.130 0.133

MgO 0.18 0.29 0.24 1.25 3.08 2.59 1.70 0.84 1.03 0.17 1.07 0.88 0.39 3.81 3.47 2.05

CaO 0.92 0.78 0.77 1.28 0.07 0.12 2.83 0.91 1.32 0.27 0.49 0.57 0.18 0.61 0.14 0.96

Na2O 2.91 2.57 2.56 2.32 0.45 0.62 3.28 2.02 1.75 3.55 0.91 1.35 0.66 0.55 0.37 0.95

K2O 5.46 5.79 6.85 3.05 2.44 3.12 0.86 1.66 2.77 4.26 1.84 6.27 3.34 0.79 1.83 2.08

Total 98.73 98.85 98.77 101.08 103.44 101.21 100.83 97.46 99.33 99.12 101.51 99.08 99.11 103.22 98.45 97.91

XMg 47.8 40.6 44.6 29.6 34.9 39.3 34.8 29.1 32.7 21.2 31.7 47.5 55.9 36.5 27.5 33.6

Quilca: QU, Mollendo: MO and Cocachacra: CO areas of the MCB; samples 242, 245 & 246: metre-scale anastomosing leucosomes containing sillimanite and rare orthopyroxene

(granitic pods or sills); sample 53a: mesosome (m); sample 53b (m): mixed mesosome-leucosome; samples 51, 55, 63 & 81: leucosomes; sample 220: stock of muscovite granite; sample 93:

aluminous gneiss with evidence for solid-melt reaction; sample 95(m): mesosome from a stromatic migmatite (see Fig. 3); sample 95: leucosome from the same migmatite; sample

209(m): aluminous gneiss, mostly comprised of mesosome from a migmatite; samples 95(m)*, 95*, 209(m)*, 242*, 245*, 246* & 220*: analysed by X-ray fluorescence others by micro-

probe scanning (courtesy A. Indares).



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Orthopyroxene

Orthopyroxene occurs in three main habits. The first type, termedOpx I, are large, mm-scale, strongly pleochroic porphyroblasts withmagnetite or quartz inclusions and is found preferentially in the

Fig. 10. AFM plot of mafic minerals from the MCB.

Table 3. Representative compositions of minerals from Camana, Quilca, Arantas, Pampa Blanca, Mollendo and Cocachacra areas.

Garnet Orthopyroxene

Sample

Fig. in text 2 CA 2 CA 132 QU 139 AR 140 AR

103 PB

19c 62 MO

93 CO

20a

93 CO

20c

Sample

Fig. in text 28 CA

28 CA

4

15 CA

19f 78 MO

SiO2 38.03 39.23 38.54 39.19 39.04 37.80 40.03 38.20 37.61 SiO2 47.13 47.80 48.32 51.79

TiO2 0.01 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.00 TiO2 0.10 0.09 0.08 0.00Al2O3 21.33 22.40 22.13 22.24 22.40 21.64 22.67 21.87 21.82 Al2O3 10.33 9.80 9.48 5.95

Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 Cr2O3 0.04 0.02 0.00 0.01

FeO 28.02 20.74 20.54 22.68 20.55 23.41 18.05 18.20 17.87 FeO 16.87 17.39 16.85 13.32

MnO 1.54 3.06 4.38 1.62 3.31 7.19 1.01 8.91 12.50 MnO 0.98 1.00 0.82 0.40

MgO 8.23 13.36 12.52 13.21 13.29 8.44 16.55 10.88 8.12 MgO 24.08 23.35 24.49 28.33

CaO 1.92 1.31 1.06 0.98 1.05 1.21 1.62 1.05 1.08 CaO 0.06 0.06 0.06 0.05

Na2O Na2O 0.00 0.02 0.02 0.00

K2O

F

Total 99.08 100.10 99.19 99.92 99.64 99.69 99.95 99.12 99.00 Total 99.59 99.53 100.12 99.85

Oxygen 12 12 12 12 12 12 12 12 12 Oxygen 3 3 3 3

Si 2.99 2.95 2.95 2.96 2.95 2.96 2.96 2.95 2.96 Si 0.85 0.87 0.87 0.92

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ti 0.00 0.00 0.00 0.00

Al 1.97 1.99 1.99 1.98 2.00 2.00 1.97 1.99 2.02 Al 0.22 0.21 0.20 0.12

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00

Fe3+ 0.04 0.06 0.06 0.06 0.05 0.05 0.07 0.05 0.02 Fe3+ 0.07 0.05 0.06 0.04

Fe2+ 1.80 1.25 1.26 1.38 1.25 1.48 1.04 1.12 1.16 Fe2+ 0.19 0.22 0.20 0.16

Mn 0.10 0.20 0.28 0.10 0.21 0.48 0.06 0.58 0.83 Mn 0.02 0.02 0.01 0.01Mg 0.96 1.50 1.43 1.49 1.50 0.98 1.82 1.25 0.95 Mg 0.65 0.63 0.66 0.75

Ca 0.16 0.11 0.09 0.08 0.09 0.10 0.13 0.09 0.09 Ca 0.00 0.00 0.00 0.00

Na Na 0.00 0.00 0.00 0.00

K K

XAlm 0.59 0.41 0.41 0.45 0.41 0.49 0.34 0.37 0.38 XMg 0.65 0.69 0.69 0.77

XPrp 0.32 0.49 0.47 0.49 0.49 0.32 0.60 0.41 0.31

XSps 0.03 0.06 0.09 0.03 0.07 0.16 0.02 0.19 0.27

Camana: CA; Quilca: QU; Arantas: AR; Pampa Blanca: PB; Mollendo: MO; Cocachacra: CO areas. Garnet: 2 CA: high-Mg garnet; secondary low-Mg garnet; 132 QU: garnet surrounded by

plagioclase; 139, 140 AR: garnet from leucosome; 103 PB: garnet surrounded by cordierite (see Fig. 19c); 62 MO: garnet from leucosome; 93 CO: pseudomorphs of a spessartine-rich

garnet (see Fig. 20c). Orthopyroxene: 28 CA: porphyroblast of high-Al orthopyroxene; 15 CA: high-Al orthopyroxene porphyroblast from a leucosome; 78 MO: low-Al orthopyroxene rim

around large secondary biotite (see Fig. 19f). Sapphirine: 28 CA: sapphirine in contact with magnetite and plagioclase; 206 CO: sapphirine pseudomorphs after sillimanite. Cordierite:

103 PB: cordierite moats around garnet (see Fig. 19c); 93 CO: cordierite in a cordierite-sapphirine symplectite (see Fig. 20b). Biotite: 140 AR: large biotite in contact with garnet; 78 MO:

biotite surrounded by a rim of orthopyroxene (see matching Opx analysis and Fig. 19f). Spinel: 2 CA, 28 CA, 85 MO: small spinel in inclusion or in contaccontact with oxide.

Fig. 9. Weighted histogram of data for Sample 28. Each small

bell-shaped curve represents the probability density function forone measurement (age and standard deviation). The dashedcurve weighted histogramis the sum of all individual bell-shaped curves. The central curve represents the ages calculatedby the statistical procedure of Montel et al. (1996). Yaxis isdimensionless.



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leucosomes. The second type, termed Opx II, are small, a few tenthsof a mm, inclusion-free grains, either isolated in the leucosome wherethey are not fundamentally different from the previous type exceptfor the size, or rims around biotite, cordierite or garnet (Fig. 19d,f).The third type, also termed Opx II without any implication of con-temporaneity with the previous type occurs as symplectites, usuallywith cordierite. Orthopyroxene (Opx I & II; Table 3) are enstatite-rich (en60 to en82; Fig. 10). On a regional basis, the enstatitecontent (or XMg MgMg + Fe

2+) of orthopyroxene increasesfrom Camana to the Mollendo area (Fig. 12), a variation thatcorrelates with that of pyrope in garnet (Figs 11 & 17). Most

orthopyroxene from the MCB is highly aluminous, Al2O3 contentranging from 4.27 to 10.33%, which results in Mg-tschermak contentranging from 17 to 35%. Some large orthopyroxene (Opx I) arezoned in Al2O3, from about 10% (XAl> 0.19) in the core to about7.5% (XAl < 0.18) towards the margins (Fig. 13). The highest Al2O3values are found in porphyroblastic orthopyroxene I from theleucosomes in the Camana area (sample 28: 10.33%), the Mollendoarea (sample 85: 10.06%) and the Cocachacra area (sample 95,10.13%). The lowest Al2O3 values are from small secondarypyroxenes growing at the expense of biotite (e.g. Mollendo area,sample 78: XMg 0.82; Al2O3 5.656.95 wt%) or from pyroxene

Table 3. (Contd)

Sapphirine Cordierite Biotite Spinel

Sample

Fi g. i n t ext 2 8 CA 2 06 C O

Sample

Fig. in text

140 AR

20c

103 PB

20c 93 CO

Sample

Fig. in text 140 AR

78 MO

19f

78 MO

19f

Sample

Fig. in text 2 CA 28 CA 85 MO

SiO2 11.59 13.46 SiO2 48.40 49.65 46.35 SiO2 38.12 37.90 38.28 SiO2 0.00 0.00 0.23

TiO2 0.00 0.04 TiO2 0.00 0.00 0.00 TiO2 2.58 2.53 3.25 TiO2 0.28 0.01 0.03

Al2O

3 62.01 59.12 Al

2O

3 31.17 31.68 34.05 Al

2O

3 16.71 16.28 15.61 Al

2O

3 60.26 63.49 60.87

Cr2O3 0.22 0.09 Cr2O3 Cr2O3 Cr2O3 0.27 0.50 0.87

FeO 9.39 10.14 FeO 2.45 2.43 2.43 FeO 8.49 6.57 6.87 FeO 14.61 17.28 19.27

MnO 0.38 1.09 MnO 0.16 0.17 0.52 MnO 0.08 0.01 0.00 MnO 0.04 1.13 0.07

MgO 15.16 15.85 MgO 12.03 11.91 11.12 MgO 19.14 21.07 20.48 MgO 5.49 13.66 11.07

CaO CaO CaO 0.00 0.01 0.01 CaO 0.01 0.00 0.00

Na2O Na2O 0.10 0.03 0.08 Na2O

K2O

F

0.15

9.78

0.787

0.13

9.83

1.24

0.14

10.22

1.17

ZnO 19.65 4.03 7.10

Total 98.75 99.79 Total 94.31 95.87 94.55 Total 95.84 95.57 96.03 Total 100.61 100.10 99.51

Oxygen Oxygen 18 18 18 Oxygen 11 11 11 Oxygen 4 4 4

Si 0.71 0.82 Si 5.06 5.10 4.85 Si 2.76 2.75 2.77 Si 0.00 0.00 0.01

Ti Ti 0.00 0.00 0.00 Ti 0.14 0.14 0.18 Ti

Al 4.46 4.23 Al 3.84 3.83 4.20 Al 1.43 1.39 1.33 Al 2.00 1.97 1.95

Cr 0.01 0.00 Cr Cr Cr 0.01 0.01 0.02

Fe3+ 0.16 0.18 Fetot 0.21 0.21 0.21 Fetot 0.51 0.40 0.42 Fe3+ 0.00 0.03 0.04

Fe2+ 0.31 0.33 Fe2+ 0.34 0.36 0.40

Mn 0.02 0.06 Mn 0.01 0.01 0.05 Mn 0.00 0.00 0.00 Mn 0.00 0.03 0.00

Mg 1.38 1.44 Mg 1.88 1.82 1.73 Mg 2.07 2.28 2.21 Mg 0.23 0.54 0.45Ca Ca Ca Ca 0.00 0.00 0.00

Na Na

K

0.02 0.01 0.02 Na

K

0.905 0.909 0.944 Zn 0.41 0.08 0.14

XMg 0.81 0.81 XMg 0.90 0.90 0.89 XPhl 71.03 77.26 76.24 XsplXher

Xgah

0.23

0.35

0.42

0.54

0.36

0.08

0.45

0.40

0.14

Fig. 11. Almandine-pyrope-(spessartine + grossularite) plot

of garnet from the MCB.

Fig. 12. XAlM1 vs. XMgplot for orthopyroxene of the MCB;

all but sample 2 are sillimanite bearing.



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from the Camana area (sample 2: XMg 0.65, Al2O3 4.27 wt%).We note that Al content tends to decrease with increasing XMgin theorthopyroxene. Compared to orthopyroxene from other UHT areasworldwide, those of the MCB are richer in Fe3+, as calculated fromcharge balance. Fe3+Fetot ranges from 0.15 to 0.25 and in a samplefrom the Mollendo area (sample 62) almost 50% is Fe 3+(Table 4),attesting to the extent of the paired substitution: Fe2+ + SiFe3+ + Al. Finally, MnO reaches 2.7% in pyroxene coexisting with

spessartine-rich garnet of the Cocachacra area, whereas Na and Cacontents are very low in all types of orthopyroxene.

Sapphirine

Two main types of sapphirine are found in the MCB. The first typecomprises relatively large sapphirine (up to a few tenths of a mm)associated with Fe-Ti oxides and separated from quartz by a thin

plagioclase aureole. This sapphirine is rare and occurs mainly inaluminous gneisses of the Camana area and its condition of forma-tion is still unclear. The second type are smaller sapphirine occurringin symplectites together with cordierite and orthopyroxene, insamples showing evidence for garnet or orthopyroxene breakdown(essentially in mesosomes from the Cocachacra area). It also occursas dactylitic intergrowths stemming from corroded sillimanite. Allthe analyzed sapphirine (Table 3) is relatively Fe-rich (0.79 3.5% in the Camana, Quilca and Pampa Blanca areas to


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sample 79, 206) of the Mollendo and Cocachacra areas contain large,subrectangular grains with an oblique extinction, identified askyanite on the basis of Raman spectra. Kyanite has been identifiedoccasionally from UHT terranes in India and Scotland (Raith et al.,1997; Baba, 1999). Analyses of a few sillimanite grains have givenabout 0.1 Fe3+ (charge balance calculation on a 20 O basis).

Spinel and Fe-Ti oxidesSpinelis rare, preferentially found in themesosomes whereit is usuallyincluded within or juxtaposed to magnetite or hemo-ilmenite.According to textural criteria, spinel most l ikely results from granularexsolution of a more complex Al-Fe-Mg-Zn-Ti-Mn-Cr solid solution.In some cases, it contains exsolutions of magnetite (sample 95) orcorundum. Occasionally, it occurs as symplectite within biotite.Where it is not entirely entrapped within magnetite, spinel may be incontact with sillimanite, plagioclase or garnet, but not with quartz.Analyzed spinel (Table 3) are hercynite-spinel-gahnite solid solutionswith low concentrations of galaxite (MnO ranging from 0.02 to 1.13wt%) and very lowconcentration of picotite (Cr2O3 ranging from0.16to 0.87 wt%). The hercynite-spinel ratio ranges from 29 to 64 withvariable but relatively high amounts of gahnite (from 5 to 42%) and atendency for gahnite to be incorporated in hercynite-rich phases.These gahnite values are well above average for both granulite facies

and UHT rocks. Reintegration of a pre-exsolution spinel compositionis hampered by the lack of readily interpretable textures. However,according to the modal proportions of magnetite and Al-spinel, theoriginal phase must have been oxidized and iron-rich ( XMag> 0.5)with minor amounts of Mg, Zn and Ti.

The general pattern from the above analytical data is, XMgCrd XMgBt XMgOpx XMgSpr > XMgGrt (Fig. 10). Unfortunately,spinel composition being dependent upon the amount and nature ofexsolution, its actual XMgmay not be representative ofXMgat peakconditions. It should be noted that the close partitioning of Fe andMg between several phases is likely to favour degenerate reactionsthus, giving a false impression of high variance of the system.

Fe- Mg P AR T I T I O NI NG B ET WEEN GAR NETAND O R T HO P Y R OXENE AND Al- CO NT ENT

O F O R T HO P Y R OX

E NEGarnet and orthopyroxene are ubiquitous Fe-Mg minerals in thealuminous (Sil-bearing) migmatites of the MCB. Owing to the pos-sibility of Fe-Mg exchange between these two minerals, KDGrt-Opx(calculated as (FeMg)Grt(FeMg)Opx from core compositions) issensitive to temperature and may be used as a semiquantitativeapproach of temperature variations in the area. Average KD valuesfor rocks of the MCB range from 2.01 to 2.82 with the lowest values(highestT) recorded in the dry rocks of the Mollendo area, highestvalues (lowest T) being found in samples from the Arantas andCocachacra areas (Table 5). Likewise, the high alumina content oforthopyroxene from the MCB is probably the result of a high tem-perature of crystallization, and as Al2O3 increases with temperature(e.g. Harley, 1998b), a reverse KDvs. Al2O3relationship is expected.However, in the orthopyroxene analyzed from MCB rocks, Al2O3

varies from about 510 wt% independently ofKDGrt-Opx,evidence forthe decoupling of KD and alumina contents already noted in otherUHT rocks (e.g. Harley, 1998b). At the thin section scale, the XMgoforthopyroxene is constant, whereas the Al2O3 varies by as much as3 wt%. Moreover, the lack of correlation betweenKDand Al contentof orthopyroxene may be related to the type of pyroxene. Forinstance, samples 62 and 74 (Mollendo area) have the lowest KDvalues and thus potentially the highest temperature of equilibration.However, these samples are characterized by small granoblasticpyroxene (Opx II) notably less aluminous than the porphyroblasticaluminous pyroxene found in the leucosomes from the same area.Rather than diffusion, these variations in Al2O3 may reflect crystal-lization of orthopyroxene during changing temperature conditions.On the contrary, the quasi-constant XMg (Fig. 17) may reflect somekind of re-equilibration at a single temperature, the lowest KD notnecessarily representing peak temperature.

CHEMI CAL SY ST EMS AND P ET R O GENET I C GR I DS

Rocks of the MCB are usually quartz-bearing at thesample scale but a distinction is made between leuco-somes that are always quartz-bearing, and mesosomesthat are usually quartz-free. At the sample scale andparticularly in the presence of a melt fraction, the

a b

Fig. 17. AAFM vs. XMgplot of ortho-pyroxene and garnet from the five areasof the MCB.

Table 5. KD Opx-Grt and Al-content of orthopyroxene (all butsample 2 are sillimanite-bearing).

Sample FeMg grt FeMg opx Kd grt-opx Al2O3-max Al2O3-opx

CAMANA

2 1.361 0.566 2.40 6.19 5.63

28 0.93 0.401 2.32 9.80 8,38

QUILCA

132 0.911 0.384 2.37 9.58 8.6443 1.093 0.404 2.71 8.89 8.18

ARANTAS

138 1.089 0.386 2.82 8.45 7.57

139 1.017 0.420 2.42 8.76 8.19

140 0.973 0.391 2.49 8.01 7.44

MOLLENDO

53 0.74 0.301 2.46 9.06 8.26

62 0.638 0.317 2.01 8.05 7.77

74 0.665 0.327 2.03 8.14 7.65

80 0.706 0.320 2.21 9.33 8.44

85 0.713 0.309 2.31 10.06 8.72

112 0.685 0.308 2.22 8.49 7.84

113 0.684 0.307 2.23 8.52 7.46

120 0.696 0.294 2.37 8.89 7.66

122 0.771 0.303 2.54 8.28 7.14

123 0.598 0.276 2.17 8.06 7.27

124 0.665 0.299 2.22 8.65 7.62

127 0.691 0.289 2.39 7.49 6.96

COCACHACRA

93 0.985 0.365 2.70 9.48 8.91

95 0.903 0.357 2.53 10.13 8.16

95 A 0.919 0.357 2.57 8.16



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availability of quartz may have been limited during thepeak of metamorphism inasmuch as the activity ofSiO2in the melt was lower than unity. Several mineralassemblages can thus be legitimately treated as quartz-free.

Except for cordierite, the only hydrated mineral inthe migmatites and the aluminous gneisses of the MCB

is biotite. Biotite-bearing assemblages (up to 20%Bt) are preferentially found in the Camana, Quilca,Arantas and Pampa Blanca areas, where they may bejuxtaposed with anhydrous (orthopyroxene-bearing)assemblages at the outcrop, sample or thin sectionscale. In the Mollendo area, biotite is rare and occursonly in the mesosomes whereas this mineral is virtuallyabsent from rocks of the Cocachacra area. Generallyspeaking rocks of the MCB can be described as dom-inantly anhydrous with layers and pods containingsmall amounts of hydrous minerals.

The FKMASH and FMAS grids

Mineral assemblages of the migmatites from the MCBcan be represented by the KFMASH system for biotite(andor K-feldspar)-bearing rocks that contain evi-dence of partial melting (perthite-rich veins and pods)and the FMAS system for biotite-, K-feldspar-, H2O-free assemblages (essentially melt-absent, anhydrousassemblages that constitute the mesosomes). The PTgrid for the KFMASH system (applicable to the leu-cosomes) has nine potential invariant points (invariantpoints, univariant, bivariant and trivariant assemblagesare designated according to the absent phase conven-

tion), among which (Os) and (Bt) are relevant to theP-T domain of UHT metamorphism under considera-tion (Fig. 18a). These points are well constrained byexperimental work (Carrington & Harley, 1995) andthermobarometric calculations (Harley, 1998b). TheP-T grid for the FMAS system (applicable to themesosomes and to some aluminous gneisses) only

allows solid-solid equilibria and permissible mineralsare orthopyroxene, garnet, sillimanite, cordierite, spineland sapphirine ( quartz). However, two types ofpetrogenetic grids are available for the FMAS system athigh temperature. The first one (Hensen, 1971; revisedby Harley, 1998b) is valid for low fO2and is supportedboth by experimental and field evidence. The secondone (Vielzeuf, 1983; Hensen, 1986) is valid for high fO2conditions but it is less well constrained experimentally,although it has been successfully applied to some fieldoccurrences. Pending more elaborate experiments rel-ative to the influence of fO2 on phase equilibria inFMAS, the lowfO2grid (Fig. 18b) will be applied here

because it fits more adequately the observations. Uni-variant reactions emanating from invariant points (Spl)(Opx) and (Qtz) in the case of FMAS and those thatemanate from (Os) and (Bt) invariant points in the caseof KFMASH are the most relevant here.

R EACT I O N T EX T UR ES

Relatively few textures from the prograde pathare expected to have resisted the high tempera-tures attained. However, mineral assemblages thatinvolve porphyroblastic phases with extreme chemical

a b

Fig. 18. (a) PTgrid for the KFMASH system (from Carrington & Harley, 1995) with shaded fields relevant to the migmatites ofthe MCB area (numbers in circles correspond to univariant reactions; others correspond to bivariant reactions); (b) PTgridfor the FMAS system (from Hensen, 1971; modified by Harley, 1998b) with shaded areas relevant to the mineral assemblages ofthe mesosomes from the MCB.



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compositions could be considered to have crystallizednear peak conditions. Numerous textures record the

breakdown of these porphyroblasts and representarrested reactions. Although these reactions can usuallybe treated as melt-absent, it is likely that some phasesprecipitated directly from melts whereas others areproduced during back-reaction with melts. These willbe best portrayed in KFMASH system. The mostspectacular reactions take place in the mesosomes andinvolve FMAS phases (orthopyroxene, garnet, sillima-nite) back-reacting either with solids or with meltsduring high-Tretrograde events. The phases that occuras coronas, symplectites and pseudomorphs (products)include cordierite, sapphirine, biotite, orthopyroxene,plagioclase and less commonly garnet. The majority ofreaction textures described below involve the break-

down of garnet and the growth of cordierite, a situationthat indicates a pressure decrease. Tri-, bi- and uni-variant assemblages (Table 6) and the correspondingtextures will be discussed starting from peak conditions.Most of these textures have already been describedin great detail by previous authors from EasternGhats, Antarctica, Sahara, Scotland and the Alps (forreferences see Harley, 1998b).

P EAK ASSEMB LAGES

The orthopyroxene-sillimanite-K-feldspar-quartz-(melt)assemblage

Northern part of the MCB

In the Camana, Quilca and Arantas areas, quartz-perthite leucosomes from migmatites (samples 15, 25,28, 39, 239, 241) contain coarse, pleochroic, Al2O3-richorthopyroxene (XMg 0.750.79; Al2O3 up to 10.30wt%) occasionally laden with sillimanite inclusions(Fig. 19b). The absence of garnet-biotite-quartzmutual contacts suggests that the conditions werereached for incongruent melting of biotite through thediscontinuous (KFMASH) reaction,

biotite garnet K-feldspar quartz

orthopyroxene sillimanite melt; 1

a vapour-absent melting reaction with a subverticalslope (Fig. 18a) that takes place at temperatures slightlyin excess of 910 C for pressures of 1.0 0.1 GPa(Carrington & Harley, 1995). The resulting assem-blage is orthopyroxene-sillimanite-K-feldspar-quartz-( garnet)-(+ melt). Additional components like Tiand F, preferentially incorporated in biotite (Dooley &Patin o Douce, 1996), may increase the variance of thisreaction and displace the stability field of biotite towardhigher temperature. Some garnet-absent assemblagescontain potentially residual primary biotite (Bt I: sam-ple 20: XMg 0.80; TiO2 3.3 wt%; F 1.4 wt%)suggesting that reaction (1) was arrested after exhaus-

tion of garnet although incomplete melting may haveoccurred according to a bivariant garnet-absent reac-tion such as:

biotite sillimanite quartz

orthopyroxene K-feldspar melt; 2

which takes place a few tens of degrees above reaction(1). Melting might even proceed at higher temperatureafter exhaustion of quartz.

Also, in garnet-poor or garnet-absent assemblages(sample 15; Fig. 5), the presence of mm-scale oxidegrains with sillimanite inclusions suggests that Tireleased by biotite melting, together with excess Fereleased by garnet resorption, precipitated as ilmeniteand magnetite. Experiments on fluid-absent melting ofbiotite (Patin o Douce & Johnston, 1991) havedemonstrated a significant increase in FeO and TiO2inthe melt fraction with increasing temperature. In thepresent case, melting of 20% (modal) biotite contain-ing 3% TiO2releases 1.2% TiO2into the melt, if melt-rock proportions are 1:1. As Ti and Fe do not easilydissolve in silica-rich melts, they may have precipitatedin the form of Fe-Ti oxides or may have formedpockets of immiscible liquids, both processes beingfavoured by high fO2 (Naslund, 1983). Upon cooling,

Table 6. Summary of FMAS reactions and mineral assemblages for the MCB.

Area

Reactions FMAS System Figure CA QU PB AR MO CO Textures

Univariant Reactions related to (Spl) invariant point

garnet + cordierite orthopyroxene + sillimanite + quartz (Spl, Spr)

Related bivariant assemblages observed:

garnet sillimanite quartz cordierite (Spl, Spr, Spx) 19c + + + Crd moats around Grt

orthopyroxene sillimanite quartz cordierite (Spl, Spr, Grt) + Crd rims around Opx in Qtz Sil-bearing rocks

garnet quartz orthopyroxene sillimanite (Spl, Spr, Crd) 19d, e + checkerboard Opx-Sil around Grt

garnet quartz orthopyroxene cordierite (Spl, Spr, Sil) 20e + Grt relict in an Opx-Crd symplectite

orthopyroxene + sillimanite garnet + sapphirine + quartz (Spl, Crd)

Related trivariant assemblage observed

garnet-sapphirine-quartz 20a + Corroded Grt in contact with Spr in Qtz matrix

orthopyroxene + cordierite + sillimanite sapphirine + quartz (Spl, Grt) not observed

orthopyroxene + sillimanite garnet + sapphirine + cordierite (Spl,Qtz) 20d + Spr in a Crd aureole around Sil in contact with Opx

Related bivariant assemblages observed

ort hopyroxene sil limanite sapphiri ne cordi erite (Spl, Qtz, Grt) 20b, c + Spr-Crd symplectite in contact with corroded Sil

garnet orthopyroxene sapphirine sillimanite (Spl, Qtz, Crd) 20c, e + Relict Grt in an Opx-Crd symplectite



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a b c

d e f

Fig. 19. (a) Secondary biotite (Bt II) resulting from melt precipitation (sample 2); (b) sillimanite inclusions in primary orthopyroxene(Opx I; sample 239); (c) cordierite moat around garnet (sample 103); (d) orthopyroxene (Opx II)-sillimanite at the contact betweengarnet and quartz (sample 112); (e) back-scattered image of orthopyroxene (Opx II) and sillimanite symplectite including aresorbed garnet initially in contact with quartz (sample 122); (f) low-alumina orthopyroxene (Opx II) around secondary biotite,itself resulting from melt precipitation (sample 78).

a b c

d e f

Fig. 20. (a) sapphirine-quartz-garnet assemblage (back-scattered images; sample 93); (b) sapphirine-cordierite symplectite formedat the expense of sillimanite and orthopyroxene (sample 93); (c) spessartine-rich garnet in a cordierite-orthopyroxene (Opx II)symplectite (sample 93); (d) cordierite ( sapphirine) reaction rims between an orthopyroxene (Opx I) porphyroblast and matrixsillimanite, and with included sillimanite (sample 95); (e) garnet relict in the core of an orthopyroxene (Opx II)-cordierite symplectitesurrounded by an orthopyroxene (Opx II) rim, pseudomorphs of a primary garnet in a sillimanite-rich mesosome (sample 95);(f) cordieriteK-feldspar symplectite, probably representing melt precipitate, surrounding corroded garnet (white line underliningthe initial shape of garnet) (sample 93).



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after the crystallization of sillimanite, TiO2 combinedwith FeO to produce ilmenite and magnetite formingan interprecipitate around sillimanite prisms. Theresulting oxide-sillimanite-rich pockets rather thanbelonging to mesosomes or melanosomes, may thus bepart of the leucosome. Otherwise, the absence of garnetfrom magnetite-rich layers (Fig. 3) could result from

the instability of the almandine component underhigh fO2, but the equivalent spinel-magnetite-quartzassemblage has not been observed. Whether due tomelting reactions or to oxidation there is a tendencyfor garnet to be consumed in high-grade assemblagesof this area and the presence of quartz, plagioclase andsillimanite inclusions in garnet shows that it does notresult from the prograde destabilization of ortho-pyroxene in the presence of sillimanite. Consequently,biotite-garnet-sillimanite-K-feldspar-quartz-(melt) isthe likely precursor of many orthopyroxene-sillimanite-K-feldspar-quartz-( garnet)-(melt) assemblages.

Southern part of the MCB

In the Mollendo and Cocachacra areas, quasi-exhaustion of biotite through reactions (1) and (2) leadsto the ubiquitous bivariant (KFMASH) assemblageorthopyroxene-sillimanite-K-feldspar-quartz-( garnet)-(+ melt). Kyanite, found locally and considered as arelict phase, is inherited from a premelting progradestage or from initial melting in the biotite-kyanitefield whereas sillimanite inclusions in porphyroblasticorthopyroxene are compatible with protracted meltingin the sillimanite field culminating with reactions (1)and (2). Porphyroblastic orthopyroxene (Opx I) hasAl2O3 contents ranging from 7.49 to 10.6 wt%, and

XMgranging from 0.78 to 0.83, whereas garnet has anXMgranging from 0.58 to 0.64. Since KDGrt-Opxhas thelowest regional values (2.012.54; Table 5), it is likelythat the highest temperatures were attained in theMollendo area. Moreover, the highXMgin both garnetand orthopyroxene is an indication that peak pressuresalso were probably attained in these areas (see Harley,1998b). The upper limit of pressure is provided by theintersection of reaction (1) with the kyanite-sillimaniteinversion curve, namely about 1.2 GPa (Fig. 18a; seealso Fig. 21a).

In most samples of the Mollendo area, ortho-pyroxene does not show any sign of instability evenwhere in direct contact with sillimanite. Sample 74contains small (recrystallized?) orthopyroxene, withlow to average Al-content (Al2O3 6.138.14 wt%)and XMg 0.7880. The corresponding KDGrt-Opx(2.03) has the lowest value for the whole area. If thisorthopyroxene is secondary, low KD may represent avalue corresponding to a lower than peak temperature.In quartz-rich aluminous granulites from the Cocach-acra area (sample 218), sillimanite prisms included inlarge quartz grains, suggest that associated ortho-pyroxene (< 0.3 mm) crystallized via melt-absent,quartz-forming, univariant (FMAS; Spl, Spr) reaction,

cordierite garnet orthopyroxene sillimanite

quartz;

3Crossing this univariant boundary (Fig. 18b) implies anegative DV and thus a pressure increase. This bound-ary is the upper limit of prograde cordierite-bearingassemblages and the resulting orthopyroxene-sillima-nite-quartz assemblage is interpreted as a relict of themelt-absent part of the prograde path in this sample.

Finally, a unique occurrence of the assemblagesapphirine-quartz is found in a biotite-free rock, 10 kmwest of Cocachacra (sample 93). The coexistenceof sapphirine and quartz is an index of extreme

a

b

Fig. 21. (a)PTpaths for migmatites of the MollendoCamanablock, interpreted from the KFMASH grid of Carrington &Harley (1995); (b)PTpaths for melanosomes of the MollendoCamana block, interpreted from the FMAS grid of Hensen(1971) modified by Harley (1998b); same legend as for Fig. 18.



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conditions of metamorphism, unlessfO2conditions arevery high. Whether prograde or retrograde, the restric-ted occurrence of this assemblage means that either thePT(andfO2) conditions for its stability were attainedonly locally or that the assemblage was not preservedupon retrogression in the rest of the MCB. Accordingto textural evidence (Fig. 20a), the assemblage is

garnet-sapphirine-quartz, with evidence for garnetresorption. This trivariant assemblage indicates that aunivariant boundary involving sapphirine and quartzhas been crossed to enter the Spr-Qtz stability field. Inview of the absence of primary cordierite in the area,we favour univariant reaction (Spl, Crd),

orthopyroxene sillimanite

garnet sapphirine quartz; 4

for this boundary (Fig. 18b) rather than univariantreaction (Spl, Opx) to enter the Spr-Grt field. Bivariantreaction (Spl, Crd, Opx),

garnet sillimanite sapphirine quartz; 5

related to reaction (Spl, Crd) would account for thetextural relationships and the preservation of corrodedgarnet after exhaustion of sillimanite. It should benoted that this reaction, cannot be precisely deter-mined due to the imprecision of sapphirine thermo-dynamic data, but, because of its positive DV it may berelated to a pressure decrease.

The cordierite-garnet-sillimanite-quartz assemblage

Some 15 km inland and 1200 m above sea level, in thePampa Blanca area, orthopyroxene is absent and none

of the observed textures suggest that it was once pre-sent. The univariant boundary corresponding to reac-tion (3) was probably not crossed during the progradepath and the peak assemblage in aluminous gneissesand biotite-rich migmatites of this area is garnet-sillimanite-quartz-( cordierite).

R ET R O GR ADE ASSEMB LAGES

Cordierite growth after garnet

In the mesosomes of quartz-rich, biotite-poor migma-tites and aluminous gneisses from the Camana, Quilcaand Pampa Blanca areas (Fig. 19c; samples 102, 103),cordierite porphyroblasts (XMgCrd 0.880.89) sur-rounding lobate garnet (XMg 0.420.46) probablyoriginated through the (FMAS) bivariant reaction,

garnet sillimanite quartz cordierite: 6

Cordierite moats around garnet are not accompaniedby K-feldspar nor biotite and thus probably formedthrough (FMAS) solid-solid reaction (6) rather thanfrom a (KFMASH) garnet + (melt) reaction. They arethus the likely product of pressure decrease. The H2Ocontent of cordierite is close to 1% (Fig. 15) and, as

free water is not expected to be present in partiallymolten rocks, H2O of the cordierite must, at somestage, have been in equilibrium with that from the melt.According to experimentally determined partitioningof water between cordierite and melt (Carrington &Harley, 1996), the H2O content of the melt was prob-ably around 4%, provided equilibrium was attained

and the fluid content of cordierite has not been alteredby subsequent events. Melting thus occurred understrongly water-undersaturated conditions.

Secondary cordierite also formed in orthopyroxenebearing rocks. In an orthopyroxene-sillimanitemigmatite (XMg-Opx 0.750.79; Al2O3 6.478.45wt%) from the Arantas area (sample 140), along theboundary with a perthite-rich leucosome, and adja-cent to cordierite-plagioclase-quartz symplectites,cordierite rims (XMg 0.90) around orthopyroxeneappears to result from the bivariant (FMAS) reaction,

orthopyroxene sillimanite quartz

cordierite; 7

which probably represents the destabilization of thetrivariant assemblage orthopyroxene-sillimanite-quartz under pressure decrease. In the same sample,garnet (XMg 0.470.49) is rare and occurs in the tri-variant (cordierite-free) subassemblage garnet-sillima-nite-quartz, which suggests that garnet composition isvery close to bulk XMg for the corresponding layer.The above assemblages are related to univariant(FMAS; Spl, Spr) reaction (3) and are compatible witha pressure decrease at constant temperature (Fig. 18b).We note that Al2O3 is high in orthopyroxene grainsthat have probably preserved their high-T aluminacontent, as their involvement in cordierite-formingreactions does not involve temperature decrease.

Plagioclase rims around garnet

In cordierite-free rocks (Camana, Quilca and Arentasareas) that contain the assemblage garnet-biotite-silli-manite-quartz, grossularite-poor garnet (Xgrs 0.03) isrimmed by oligoclase-andesine feldspar (sample 16, 20,23, 27, 28, 29, 41). As garnet does not show evidence ofresorption in these rocks, such rims are interpreted asdue to the breakdown of the sole grossularite compo-nent. The corresponding reaction:

grossularitein garnet sillimanite quartz anorthitein plagioclase 8

could be due to either an isothermal decompression oran isobaric heating. The first path is more plausible inview of the decompression-associated breakdown ofgarnet into cordierite observed in the same areas.

Orthopyroxene recrystallization

In rocks that contain the assemblage sillimanite-orthopyroxene-K-feldspar-quartz-( garnet), primary



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Al-rich orthopyroxene may have recrystallized intoAl-poor orthopyroxene according to the reaction,

Al-orthopyroxene I garnet quartz

plagioclase orthopyroxene II: 9

This reaction may in part account for the variation of

both the Al content of orthopyroxene and modalproportion of garnet in similar assemblages fromnearby areas.

Orthopyroxene-sillimanite incompatibility

In biotite-free rocks of the Cocachacra area (samples93 & 95), the breakdown of stubby orthopyroxene(Opx I) gave rise to cordierite-sapphirine symplec-tites (Fig. 20b), whereas dactylitic sapphirine (Spr II)preferentially grew at the expense of sillimanite(Fig. 20b,c). These textures suggest the breakdown ofpeak orthopyroxene reacting with sillimanite to pro-duce sapphirine and cordierite through the bivariant

(FMAS) reaction:


sapphirine cordierite 10

which is related to the (Spl, Qtz) univariant (FMAS)reaction (Fig. 18b):


garnet sapphirine cordierite: 11

The absence of garnet as a breakdown product oforthopyroxene suggests that the univariant (Spl, Qtz)boundary was not crossed, although newly formed

garnet may have reacted with sillimanite and SiO2-richmelts to form cordierite and sapphirine. This mayaccount for the occurrence of minute sapphirine crys-tals in large cordierite (Fig. 20d; samples 95 & 140).Even if univariant boundary (11) is intersected, garnetmay be absent due to the instability of the almandinecomponent at relatively high fO2, which, in turn,accounts for the presence of fine magnetite inclusionsin the cordierite.

Garnet breakdown in SiO2-saturated assemblages

In quartz-rich samples from the Mollendo area, relict

garnet (sample 112:XMg 0.60, Fig. 19d; sample 122:XMg 0.58; Fig. 19e) is rimmed by a first moat ofplagioclase, itself surrounded by Al-poor orthopyrox-ene (sample 112: XMg 0.82; Al2O3 6.95 wt%;sample 122: XMg 0.82, Al2O3 6.118.28 wt%).Garnet breakdown can thus be summarized as,

garnet quartzor SiO2 from melt

orthopyroxene plagioclase; 12

a reaction that involves a volume increase consistentwith a pressure decrease. The grossularite component

of reacting garnet provides anorthite to the productplagioclase, whereas the albite component more likelycomes from the melt. It is worth noting that in thesame samples, the boundaries of quartz inclusionswithin porphyroblastic garnet do not show any sign ofreaction. Whereas the volume increase consecutive togarnet breakdown is readily accommodated if the

reaction products crystallized in contact with a melt,the same does not happen in trapped inclusions.

Secondary, relatively Al-poor orthopyroxene (Opx II;Sample112:Al2O3 6.958.49wt%; XMg 0.800.87)is also formed in checkerboard-like orthopyroxene-sillimanite symplectites (Kriegsman & Schumacher,1999) and preferentially grew at the expense of garnet(XMg 0.60), in quartz-present domains (Fig. 19d,e).The bivariant (FMAS) reaction,

garnet quartz orthopyroxene sillimanite; 13

responsible for these textures is related to the univar-iant (Spl, Spr) reaction (Fig. 18b) and is also favoured

by a pressure decrease.In biotite-free, quartz-saturated rocks from the

Cocachacra area, mm-scale garnet from an early gar-net-orthopyroxene-sillimanite-quartz assemblage isalmost completely replaced by either of the followingtwo types of pseudomorphs. In sample 95, an ortho-pyroxene (Opx II)-cordierite ( plagioclase-magnet-ite) symplectite, itself surrounded by a partial rim oforthopyroxene (Opx II) ( plagioclase-magnetite) isdeveloped only where initial garnet was in contact withquartz or with the leucosome (Fig. 20e). In the core ofthe symplectites, spessartine-rich garnet relicts (Sp10)are present whereas the Mn content of orthopyroxeneII is about 1.17 wt%. In the groundmass, sillimanite isseparated from primary, Mn-poor (0.4 wt%) ortho-pyroxene (Opx I) by a fine rim of cordierite withmagnetite droplets and vermicular sapphirine (seereaction (10) above). These complex textures can beexplained by the following succession of bivariant(FMAS) reactions,

orthopyroxene sillimanite quartz

cordierite; 14

garnet quartz orthopyroxene cordierite; 15

which are related to the univariant (FMAS; Spl, Spr)reaction (3).

Crossing this boundary implies a major volumeincrease and is thus favoured by a pressure decrease(Fig. 18b). The sapphirine vermicules found in the cor-dierite may be explained if, in an orthopyroxene-sillimanite aggregate from which quartz has beenexhausted due to reaction (14), further reaction proceedsunder decreasing pressure according to reaction (10),which implies crossing the univariant boundary (11).

In sample 93, a cordierite-sapphirine-plagioclase-( magnetite orthopyroxene) symplectite is obser-ved. In this case, the pseudomorphs of primary garnet



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consist of cordierite with sapphirine vermicules( plagioclase magnetite). Spessartine-rich garnetrelicts (Sp20; Fig. 20c) occur in the centre of the sym-plectites, whereas an Mn-rich orthopyroxene (Opx II;MnO 2.70%) forms a partial rim around the sym-plectite. In this case, garnet breakdown probably tookplace under silica-undersaturated conditions according

to the bivariant (FMAS) reactions,

garnet sillimanite sapphirine cordierite; 16

garnet orthopyroxene sapphirine

sillimanite; 17

both being related to univariant reaction (Spl, Qtz).Crossing this univariant boundary in the sense ofsapphirine is also compatible with a pressure decrease(Fig. 18b; see also Fig. 21b).

It should be noted that although these reactions arevalid for silica-undersaturated systems, quartz is pre-sent in the sample, but it is likely that at the onset of

decompression much if not all of it was incorporated inthe melt within which silica activity was less than unity.This illustrates the need for considering mm-scalesubassemblages, since reactions that should only occurin silica-undersaturated solid rocks can be observed inmelt-present silica-saturated rocks. Finally, the pres-ence of submicroscopic spinel grains within newlyformed cordierite (samples 95, 93) suggests crossing ofthe (Opx, Qtz) boundary under still declining pressure.As spinel does not occur together with quartz, theunivariant boundary (Opx, Spr) was not intersectedwhich implies an inflection of the decompression path(Fig. 18b; see also Fig. 21b).

B ACK - R EACT I O NS I NVO LVI NG MELTAND MELT P R ECI P I T AT I O N

Several textures are strongly indicative of solid-meltreactions or melt precipitation. In the Camana area,the modally limited but ubiquitous subassemblagebiotite-quartz( plagioclase) suggests coprecipitationfrom silicate melts, and as resorbed garnet is oftenassociated with this texture (Fig. 19a; Sample 2), itmay be involved in (KFMASH) bivariant reaction,

garnet orthopyroxene melt

biotite plagioclase quartz: 18

Alternatively, garnet may be a product of melt preci-pitation. For instance, the orthopyroxene-poor, silli-manite-free quartzofeldspathic sample 2 contains twogenerations of garnet (Fig. 17): a pyrope-rich garnet(XMg 0.53) is found isolated in the matrix as is theorthopyroxene (Al2O3 4.36.2 wt%; XMg 0.650.68), whereas a pyrope-poorer garnet (XMg 0.39) isassociated with quartz-biotite-plagioclase symplectitesin microdomains that may represent melt segregations.The pyrope-poor garnet and the biotite could be con-sidered as products of melt precipitation according to

reaction (1) operating backwards until exhaustion ofsillimanite (isobaric cooling). In the case of sample 2however, considering the significant difference in thepyrope content of garnet, it is more plausible to invokea lowerT- lowerP discontinuous (KFMASH) reactionsuch as,

cordierite orthopyroxene melt biotite II garnet II K-feldspar quartz; 19

that proceeded until exhaustion of cordierite (Figs. 18a& 21a). This evolution presumes that either cordieritewas present in peak assemblages in the case wherepressure was not higher than about 1.0 GPa (seeHarley, 1998b) or more likely that primary garnet waspreviously converted into cordierite during the firststages of decompression.

In a quartz-saturated sillimanite-orthopyroxenemigmatite of the Arantas area, interstitial, mm-scale,myrmekite-looking symplectites of cordierite-plagio-clase-quartz can be confidently interpreted as due to

melt precipitation. As neither K-feldspar nor biotiteare involved in the assemblage, the reaction probablytook place between K-poor melt, sillimanite andorthopyroxene.

In the Cocachacra area (sample 93a), mm-scalemyrmekite-looking, vermicular intergrowths of cordi-erite and K-feldspar surrounding corroded garnet inquartz-bearing rocks (Fig. 20f) that could be mis-interpreted as retrograded osumilite, result from thecontinuous solid-melt reaction,

garnet sillimanite melt

cordierite K-feldspar: 20

This reaction is sensitive to theXMgof garnet and, duethe relatively flat MgMg + Fe isopleths, the insta-bility of garnet in the presence of melt will be favouredby a pressure decrease (Spear et al., 1999).

SECO NDAR Y MELT I NG

Renewed melting related to a second stage ofdecompression is found in several samples from theMollendo area (e.g. sample 78), where large biotite(Bt II; XMg 0.85; TiO2 2.92 wt%; F 1.2 wt%)may be overgrown by a relatively Al-poor ortho-pyroxene (Al2O3 6.2 wt%) forming a partial corona

between biotite and either quartz or the leucosome(Fig. 19f). This texture could be explained by a fluid-absent (KFMASH) multivariant melting reactionrelated to the (Os, Sil) univariant reaction (19). Thislate melting event took place after major decompres-sion and cooling from ultra-high temperatures.

P HY SI CAL CO NDI T I O NS O F MET AMO R P HI SM

The identification of mineral assemblages and texturalrelationships together with the compositions of Fe-Mg



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phases allow a semiquantitative evaluation of thephysical conditions and their evolution during themetamorphism and migmatization of the MCB.Pendingthe determination of reliable PTestimates, peak andpostpeak PTevolution can be monitored using reac-tion textures and appropriate PTgrids (Fig. 21).

Water content of assemblages

The proportion of hydrous minerals (essentiallybiotite and cordierite) within rocks of the MCBvaries from 20 to almost 0%. Several samples fromthe Mollendo and Cocachacra areas are devoid ofhydrous minerals whereas migmatites from Quilcamay contain up to 20% biotite. It is likely thatamong the biotite, the F-rich ones may have sur-vived melting while F-poor ones precipitated frommelts. The distribution of water within the assem-blages was thus variable throughout the area, but itis likely that, at least in the south-eastern part of the

MCB, water was only present as a component of themelt at the peak of metamorphism. Cordierite is ableto trap water from the melt with a melt-solid parti-tion coefficient of about 4, as experimentally deter-mined by Carrington & Harley (1996). In PampaBlanca and Cocachacra areas, the H2O content ofcordierite is about 1%, which means that cordieritemay have been in equilibrium with a melt containingabout 4% H2O.

Oxygen fugacity

Oxygen fugacity is difficult to estimate in rocks fromthe MCB because of extensive exsolution-re-equili-

bration of Fe-Ti oxides during cooling. Textural evi-dence, however, attests to the presence, during peakmetamorphism, of an ilmenitess-magnetitess buffer inmost assemblages. In the Pampa Blanca area, mag-netite (Xmag 0.69,XuSpl 0.30) coexists with ilmenite(Xilm 0.80, Xhem 0.20) which corresponds to a log

fO2 ) 11 at 950 C (Ni-NiO buffer; Spencer &Lindsley, 1981). However, in many samples, puremagnetite (Xmag 0.99) is found coexisting withFe-Mg spinel, in keeping with granular exsolution andre-equilibration at lower temperatures. The Fe3+

content of several silicates as obtained from charge-balance calculation also attests to relatively high fO2.Such is the case for sapphirine and orthopyroxene thathave Fe3+Fetot ratios of about 0.4 and 0.2, respect-ively (see Table 4). Sillimanite has about 0.1 Fe3+ (ona 20 O basis) substituted for Al. Finally, the absence ofrutile and graphite is in keeping with a relatively high

fO2.

Partial Melting

In Camana, Quilca, Arantas, Mollendo and Coca-chacra areas, the assemblage orthopyroxene-sillima-nite-K-feldspar-quartz-( Grt) shows that peak

temperature conditions were above those of the (Os,Crd) major boundary (Fig. 21a), and probably wellabove 900 C if maximum F and Ti enter biotite(Patin o Douce & Jonhston, 1991). Minimum pres-sures are provided by the absence of osumilite andprimary cordierite, and those of the (Bt) invariantpoint where these minerals would coexist with melt.

Local evidence of vapour-absent melting of biotite-( garnet) according to a quartz-free melting reac-tion such as,

biotite sillimanite garnet orthopyroxene

sillimanite K-feldspar melt; 21

suggests that either biotite was previously exhaustedfrom quartz-rich assemblages or that quartz wasexhausted via reaction (1) in quartz-poor assemblages.In fact, during prograde melting, temperature wasprobably buffered, in the sillimanite field, at the (Os,Crd) boundary until exhaustion of biotite in quartz-

rich rocks, and subsequent temperature increase per-mitted continuous biotite melting in quartz-free rocks.This situation may have prevailed in Mollendo andCocachacra areas where melt production has dramat-ically decreased the modal proportion of garnet andvirtually removed biotite from the assemblages. Con-sidering the very low water content of generated melts(from 0 to 4%), any pressure decrease at high tem-perature would prevent crystallization, thus favouringa continuous process of melt segregation. On the otherhand, the high viscosity of quasi-anhydrous melts isprobably responsible for the limited amount of meltmigration and for the preservation of the stromatichabit of migmatites. In that respect it is worth noting

that evidence of melt migration in the form of apliticdykes and granitic stocks (still undated but probablyrelated to the UHT event) are more abundant in thenorthern part of the MCB where water content of themelts was higher and viscosity lower.

Peak temperature and pressure

Besides extensive melting in the orthopyroxene-sillimanite field, evidence for ultra-high temperature isprovided by the high Al content of orthopyroxene.From Camana to Mollendo, Al2O3 values approach-ing 10% (corresponding to XAl of c. 0.10 on a 10 Obasis) are not exceptional and are compatible withtemperatures of about 1000 C (e.g. Harley, 1998b).Also, a unique occurrence of sapphirine in contactwith quartz in the Cocachacra area may indicatetemperatures above 1000 C (Fig. 21b). The absenceof primary (prograde) magnesian cordierite suggestspressures in excess of 1.0 GPa (e.g. Harley, 1998b).Moreover, retrograde cordierite in Pampa Blancaarea is close to pure Mg end-member, and probablygrew under already declining pressures around1.0 GPa. Finally, the presence of kyanite porphyro-blasts in the Mollendo area suggests an early stage of



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higher pressure or lower temperature metamorphism.This early kyanite must be inherited from a pre- or anearly melting stage and it is unlikely that meltingoccurred in the kyanite-orthopyroxene field, becausesillimanite is the only Al-silicate so far found asinclusions in the orthopyroxene. The vapour-absentdiscontinuous melting curve of Mg-rich biotite

(Carrington & Harley, 1995) intersects the kyanite-sillimanite inversion curve at about 920 C and1.22 GPa and at slightly higher conditions if biotite isstabilized by significant incorporation of Ti and F. Ifmelting occurred between 950 and 1050 C in thesillimanite field, pressure thus cannot have exceeded1.3 Gpa (Fig. 21a). Based on the above elements, areasonable estimate for the ubiquitous orthopyrox-ene-sillimanite-quartz-( melt) assemblage in theMCB should therefore be in the range 9001000 Cand 1.01.3 GPa. Lower conditions for the peak ofmetamorphism may apply to the Pampa Blanca areawhere no trace of the orthopyroxene-sillimanite

assemblage was found (Fig. 21b). A regional pressuregradient from Camana to Mollendo is independentlysuggested by the increasing XMg of garnet (fromabout 0.30 to over 0.60) and orthopyroxene (from0.61 to 0.84) in rocks of otherwise similar bulk XMg.Finally, it should be remembered that this UHTmetamorphism has a regional character as it occurs inan area of several thousands km2, and not as boudinsor localized outcrops as is the case for several UHTarea so far described.

The retrograde path

Within the PT range defined above for peak condi-

tions, the low fO2 FMAS grid (Hensen, 1971; Harley,1998b) is dominated by the assemblages orthopyrox-ene-sillimanite-quartz and garnet-cordierite-sillima-nite-quartz, whereas the spinel-quartz assemblage isrestricted to lower pressures and higher temperatures,namely away from the stable invariant point (Spl). Forthe same PT range, the high fO2 grid requires thepresence of stable invariant points (Spr) and (Grt) andhence the stability of the assemblage spinel-quartz.Among the observations suggesting that the low fO2grid is more appropriate for UHT rocks of the MCBwe note that: (a) garnet was stable during peak con-ditions even if it has been largely consumed by redoxand melting reactions; (b) neither spinel-quartz assem-blages, nor garnet andor sillimanite coronas aroundspinel have been observed in any of the above areas,suggesting that spinel never coexisted with quartz; (c)retrograde textures illustrated by the formation oforthopyroxene-sillimanite and orthopyroxene-cordier-ite symplectites can be accounted for by bivariant (Spl,Spr, Crd) and (Spl, Spr, Qtz) reactions attesting to thestability of the (Spl) point of the low fO2 grid; theycannot be accommodated with any of the stableinvariant points of the high fO2 grid. Thus, it seemsthat although fO2 was relatively high during UHT

metamorphism of the MCB, it was not high enoughto support an inversion in the topology of Hensensoriginal FMAS grid.

A preliminary interpretation of the retrogradetextures could stress the breakdown of garnet and theformation of cordierite, whether or not associated withsapphirine or orthopyroxene. There is unequivocal

evidence for a major initial stage of decompression athigh-T in Mollendo and Cocachacra areas. Inthe orthopyroxene-sillimanite-quartz domain, ortho-pyroxene-sillimanite symplectites grew after garnet-quartz according to reaction (13). Then the entry intothe cordierite field is marked by orthopyroxene-cordierite symplectites after garnet-quartz accordingto reaction (15). Evidence for further decompressionin low-SiO2 assemblages is provided by garnetbreakdown into fine aggregates of orthopyroxene-sapphirine-sillimanite according to reaction (17) andsillimanite-orthopyroxene giving cordierite-sapphirineaccording to reaction (11). Finally, the formation of

very fine spinel-cordierite assemblages suggests thatthe decompression path went on as to intersect the(Opx, Qtz) reaction (Fig. 21), before switching fromdecompression to cooling. Following the initial stageof decompression, isobaric cooling took place. Thisstage probably lead to P-T conditions of the (Os)invariant point as suggested by the tendency of XMgof garnet and orthopyroxene and XAl of ortho-pyroxene of the MCB to converge toward the valuesobtained by Harley (1998b) for calculated isopleths atthis invariant point.

CO NCLUSI O NS

The orthopyroxene-sillimanite-quartz-( garnet-K-feldspar) assemblage, usually considered as diagnosticof UHT metamorphism (e.g. Harley, 1998b), is ubi-quitous in the Camana, Quilca, Arantas, Mollendoand Cocachacra areas although by no means thisimplies uniform temperature throughout the area. ThePampa Blanca area, which is devoid of this assem-blage, may represent lower metamorphic conditions,and as this area is the most inland, it is likely that anorthopyroxene-sillimanite isograd runs more or lessparallel to the Pacific coast. The highest temperaturesattained in the Mollendo and Cocachacra areas mayhave been locally in excess of 1000 C at pressuresaround 1.2 GPa. Mineral assemblages and texturalrelationships suggest a three-stage retrograde P-Tpath, starting with an initial decompression at high-Tfollowed by an isobaric cooling stage and a seconddecompression. The first stage is best recorded in theCocachacra area by solid-solid FMAS reactions and ischaracterized by a quasi-isothermal decompression ofabout 0.3 GPa. The isobaric cooling stage does notproduce any typical reaction texture and is ratherinterpreted from the absence of osumilite, the absenceof spinel-quartz assemblages and the tendency for gar-net and orthopyroxene to stabilize their compositions



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Harley, S. L., 1998a. Ultrahigh temperature granulite meta-morphism (1050oC, 12 Kbar) and decompression in garnet(Mg70)-orthopyroxene-sillimanite gneisses from the RauerGroup, East Antartica. Journal of Metamorphic Geology, 16,541562.

Harley, S. L., 1998b. On the occurrence and characterization ofultrahigh-temperature crustal metamorphism. In: What DrivesMetamorphism and Metamorphic Reactions? Special Publica-

tion 138, (eds Treloar, P. J. & OBrien, P. J.), pp. 81107.Geological Society, London.Harley, S. L. & Carrington, D. P., 2001. The Distribution of

H2O between Cordierite and Granitic Melt: H2O Incorpora-tion in Cordierite and its Application to High-grade Meta-morphism and Crustal Anatexis. Journal of Petrology, 42,15951620.

Harley, S. L., Hensen, B. J. & Sheraton, J. W., 1990. Two-stagedecompression in orthopyroxene-sillimanite granulites fromForefinger Point, Enderby Land, Antarctica: implications forthe evolution of the Archean Napier Complex. Journal ofMetamorphic Geology, 8, 591613.

Hensen, B. J., 1971. Theoretical phase relations involvingcordierite and garnet in the system MgO-FeO-Al2O3-SiO2.Contributions to Mineralogy and Petrology, 33, 191214.

Hensen, B. J., 1986. Theoretical phase relations involvingcordierite and garnet revisited: the influence of oxygen

fugacity on the stability of sapphirine and spinel in the systemMg-Fe-Al-Si-O. Contributions to Mineralogy and Petrology,92, 362367.

Irwin, E., 1975. Structural evolution of the northernmost Andes,Colombia.U.S. Geological Survey, Professional Paper,846,47.

James, D. E. & Brooks, C., 1976. Preliminary Rb-Sr data on theminimum age of the central Andean Precambrian basementcomplex. Carnegie Institution of Washington Yearbook, 75,213216.

Kienast, J. R. & Ouzegane, K., 1987. Polymetamorphic Al, Mg-rich granulites with orthopyroxene-sillimanite and sapphirineparageneses in Archean rocks from the Hoggar, Algeria.Geological Journal, 22, 5779.

Kretz, R., 1983. Symbols for rock forming minerals. AmericanMineralogist, 68, 277279.

Kriegsman, L. M. & Schumacher, J. C., 1999. Petrology ofsapphirine-bearing and associated granulites from Central SriLanka. Journal of Petrology, 40, 12111239.

Litherland, M., Annells, R. N., Darbyshire, D. P. F. et al.,1989. The Proterozoic of eastern Bolivia and its relationshipto the Andean mobile belt. Precambrian Research, 43, 157174.

Montel, J. M., Foret, S., Veschambre, M., Nicollet, C. &Provost, A., 1996. Electron microprobe dating of monazite.Chemical Geology, 131, 3753.

Naslund, H. R., 1983. The effect of oxygen fugacity on liquidimmiscibility in iron-bearing silicate melts. American Journalof Science, 283, 10341059.

Pacci, D., Herve, F., Munizaga, K., Kawashita, K. & Cordani,U., 1981. Acerca de la edad Rb-Sr Precambrica de rocas de laformation Esquistos de Bele n (Departamento de Parinacota,Chile). Revista Geologica de Chile, 11, 4350.

Patino Douce, A. E. & Jonhston, A. D., 1991. Phaseequilibria and melt productivity in the pelitic system:implications for the origin of peraluminous granitoids andaluminous granulites. Contributions to Mineralogy and

Petrology, 107, 202218.Raith, M., Karmakar, S. & Brown, M., 1997. Ultra-high-temperature metamorphism and multistage decompressionalevolution of sapphirine granulites from the Palni HillRanges, southern India. Journal of Metamorphic Geology,15, 379399.

Restrepo-Pace, P. A., Ruiz, J., Gehrels, G. & Cosca, M., 1997.Geochronology and Nd isotopic data of Grenville-age rocksin the Colombian Andes: new constraints for Late Proter-ozoic-Early Paleozoic paleocontinental reconstructions ofthe Americas. Earth and Planetary Science Letters, 150,427441.

Sadowski, G. R. & Bettencourt, J. S., 1996. Mesoproterozoictectonic correlations between eastern Laurentia and thewestern border of the

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